Not Applicable
Not Applicable
This invention relates to multimedia image projection displays and more particularly to an electro-mechanical system for automatically correcting keystone distortion in such displays.
Presentations using multimedia projection display systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction. As shown in FIG. 1, a multimedia projector 10 is typically supported by a surface 11 and receives analog video signals from a cable 12 connected to a video source, such as a personal computer (xe2x80x9cPCxe2x80x9d) 14. The video signals represent still, partial-, or full-motion display images of the type rendered by the PC. The analog video signals are converted in multimedia projector 10 into digital video signals to control digitally-driven display devices, such as a transmissive or reflective liquid crystal displays or digital micro-mirror devices, to form a display image (not shown) that is cast as a projected image 16 surrounding a projection axis 18 onto a projection surface 20. If, as shown in FIG. 1, projection axis 18 is perpendicular to projection surface 20, projected image 16 will be substantially undistorted. A wide variety of such multimedia projectors are available from In Focus Systems, Inc., the assignee of this application.
Referring to FIG. 2, a necessary feature of most projectors, such as multimedia projector 10, is an elevator mechanism 22 for tilting multimedia projector 10 and/or its projection axis 18 up or down relative to surface 11 to position projected image 16 on projection surface 20. Unfortunately, this tilting causes projection axis 18 to be non-perpendicular to projection surface 20. As shown in FIG. 2, this results in trapezoidal distortion of projected image 16, which distortion is commonly referred to as keystone distortion, or keystoning, terms tracing their origins to early Keystone Corporation movie projectors.
To minimize keystoning, some prior workers have coupled the projector elevator mechanisms to various tilting mirror and lens arrangements that compensate for the keystoning. Unfortunately, such optical mechanisms are costly and prone to collecting image-obscuring dust in the optical path.
Other prior workers have employed signal processing circuits to avoid keystone correction mechanisms and their related optical path problems. For example, U.S. Pat. No. 5,664,858 for METHOD AND APPARATUS FOR PRE-COMPENSATING AN ASYMMETRICAL PICTURE IN A PROJECTION SYSTEM FOR DISPLAYING A PICTURE describes logic circuits that oppositely distort the image being projected, thereby compensating for keystoning. Yet other workers have employed the signal processing capabilities of multimedia projectors or their associated PCs to achieve similar results. An image suitable for correcting keystone distorted image 16 of FIG. 2, is shown in dashed lines in FIG. 1 as predistorted image 26. One thing these systems typically have in common is manual controls 24 that permit an operator to adjust the degree of predistortion. Manual controls 24 are adjusted until projected image 16 appears satisfactory to the eye of the operator. While manual controls 24 are usually effective in achieving an acceptably undistorted projected image 16, adjusting such controls is time-consuming and decreases the user-friendliness of multimedia projector 10, which typically includes numerous other controls as well.
What is needed, therefore, is an automatic keystone distortion correction system for multimedia projectors that requires no additional optics or manual adjustments.
An object of this invention is, therefore, to provide an automatic keystone distortion correction apparatus and method for use in multimedia projectors.
Another object of this invention is to provide a keystone distortion correction apparatus and method that requires no additional optics or manual adjustments.
A multimedia projector suitable for use with this invention includes a controller having an image processor that forms an image on an image forming device. Illumination impinging on the image forming device reflects off the image, propagates through a prism assembly and a projection lens, and along a projection axis to land on a projection surface as a projected image. An elevator mechanism tilts the projection axis relative to a supporting surface to position the projected image on the projection surface. However, the tilting of the projection axis causes keystoning of the projected image. This invention employs techniques for automatically correcting the keystoning without requiring manual operator intervention.
In a first preferred embodiment, an inclinometer is mechanically coupled to the multimedia projector to automatically generate angle data indicative of a projection axis angle. The controller receives the angle data and coacts with the image processor to predistort the image so as to compensate for the keystoning.
In a second preferred embodiment, the elevator mechanism includes an elevator shaft that is captured within a housing and is released for sliding within the housing by an operator moving an actuator that is accessible at an exterior surface of the multimedia projector. The elevator shaft further includes teeth that alternately pass and obscure light propagating through the housing as the elevator shaft undergoes movement. An optical sensor senses a number of teeth proportional to a distance that the elevator shaft moves and conveys a corresponding number of electrical signals to the controller. The electrical signals correspond to the angle data in that a counter, accumulator, register, or equivalent within the controller converts the number of pulses into data that are indicative of the projection axis angle. The electrical signals represent relative movement of elevator shaft.
In a third preferred embodiment, the teeth of the elevator mechanism are replaced by a rotary encoder wheel that is rotatably coupled to linear motion of the elevator shaft.
Alternative embodiments of the teeth may include various forms of optically readable indicia including an absolute encoder scale for generating absolute angle data that are more directly usable by the image processor for predistorting the image to compensate for the keystoning. Other suitable forms of absolute encoders include variable differential transformers and potentiometers.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceed with reference to the accompanying drawings.